Substituted N-ethylglycine derivatives for preparing PNA and PNA/DNA hybrids

ABSTRACT

There are described N-ethylglycine derivatives of the formula I ##STR1## in which PG is a urethane-type or trityl-type amino protective group which is labile to weak acids, X is NH, O or S, Y is CH 2 , NH or O, and B&#39; are bases customary in nucleotide chemistry, the exocyclic amino or hydroxyl groups of which being protected by suitable, known protective groups, or are base substitute compounds, and their salts with tert-organic bases, as well as a process for their preparation. The N-ethylglycine derivatives of the formula I are used in the preparation of PNA and PNA/DNA hybrids.

The present invention relates to novel substituted N-ethylglycinederivatives for preparing PNA and PNA/DNA hybrids as described in thesimultaneously filed application "Peptide oligonucleotide derivatives,their preparation and their use" (HOE 94/F 057, DE-P 44 08 534.6)

Peptide or polyamide nucleic acids (PNA) are DNA-analogous compounds, inwhich the deoxyribose phosphate backbone was replaced by a peptideoligomer. The syntheses hitherto described in the literature (forexample Michael Egholm, Peter E. Nielsen, Rolf H. Berg and Ole Buchardt,Science 1991, 254, 1497-1500; Ole Buchardt, Michael Egholm, Peter E.Nielsen and Rolf R. Berg, WO 92/20702) use, as a temporary protectivegroup for the amino group of the monomer, the acid-labiletert-butyloxycarbonyl (Boc) protective group, which is eliminated bymedium-strong acids such as, for example, trifluoroacetic acid. Thesolid-phase synthesis of oligomers is carried out in accordance with thecustomary peptide synthesis processes, as they have been described, forexample, by Merrifield (B. Merrifield, J. Am. Chem. Soc., 1963, 85,2149). The PNA oligomer is eliminated with the aid of a strong acid,customarily using liquid hydrogen fluoride.

The repeated treatment with trifluoroacetic acid and the subsequentcleavage using hydrogen fluoride is not compatible with the synthesis ofmixed PNA/DNA sequences, since the nucleosidic linkage is not stableunder these conditions. In particular, the purine nucleotidesdeoxyguanosine and deoxyadenosine are rapidly cleaved on theN-glycosidic linkage by strong acids. Moreover, it would be particularlydesirable for the synthesis of such molecules to use the customary DNAsynthesizers and to largely retain the chemistry used in this apparatus.This also applies to the preparation of PNA sequences with the aid ofsuch apparatus.

It is therefore an aim of the invention to provide glycine derivativeswhich allow a simple construction of PNA and PNA/DNA hybrids as well asthe use of automatic synthesizers.

Substances which are suitable for this purpose are the compounds of theformula I ##STR2## in which PG is a urethane-type amino protective groupwhich is labile to weak acids, such as, for example,1-(1-adamantyl)1-methylethoxycarbonyl (Adpoc),1-(3,5-di-tert-butylphenyl)1-methylethoxycarbonyl (t-Bumeoc) and1-methyl-1-(4-biphenyl)ethyloxycarbonyl (Bpoc),3,5-dimethoxyphenyl-2-propyl-2-oxycarbonyl (Ddz), or a trityl-type aminoprotective group which is labile to weak acids, such as triphenyl (Trt),(4-methoxyphenyl)diphenylmethyl (Mmt), (4-methylphenyl)diphenylmethyl(Mtt), di-(4-methoxyphenyl)phenylmethyl (Dmt) and 9-(9-phenyl)xanthenyl(pixyl),

X is NH, O or S, preferably NH or O,

Y is CH₂, NH or O, preferably CH₂, and

B' are bases customary in nucleotide chemistry, for example naturalbases, such as adenine, cytosine, guanine, thymine and uracil, orunnatural bases, such as purine, 2,6-diaminopurine, 7-deazaadenine,7-deazaguanine, N⁴ N⁴ -ethanocytosine, N⁶ N⁶ -ethano-2,6-diaminopurine,5-methylcytosine, 5-(C₃ -C₆)-alkynyluracil, 5-(C₃ -C₆)alkynylcytosine,5-fluorouracil and pseudoisocytosine, the exocyclic amino or hydroxylgroups of all of these being protected by suitable known protectivegroups, such as the benzoyl, isobutanoyl, acetyl, phenoxyacetyl,4-(t-butyl)benzoyl, 4-(t-butyl)phenoxyacetyl, 4-(methoxy)benzoyl,2-(4-nitrophenyl)ethyloxycarbonyl,2-(2,4-dinitrophenyl)ethyloxycarbonyl, 9-fluorenylmethoxycarbonyl,diphenylcarbamoyl or formamidine group, preferably the benzoyl,isobutanoyl, acetyl, phenoxyacetyl, 4-(t-butyl)benzoyl or4-(methoxy)benzoyl group, and also, in the case of guanine, by acombination of 2-N-acetyl with 6-O-diphenylcarbamoyl, or are basesubstitute compounds, such as, for example, imidazole, triazole ornitroimidazole,

and their salts, preferably their salts with tertiary organic bases,such as, for example, triethylamine or pyridine.

Compounds of the formula I where Y is CH₂ can be obtained, for example,by reacting a compound of the formula II ##STR3## in which PG and X areas defined above and,

R¹ is hydrogen or an ester protective group, such as, for example,methyl, ethyl, butyl or 2-(methoxyethoxy)ethyl,

with a compound of the formula III ##STR4## in which B' is as definedabove and

Y is CH₂

at 0-45° C., preferably at room temperature, in a suitable solvent, suchas, for example, DMF, acetonitrile, dichloromethane or mixtures of thesesolvents, using a coupling reagent conventionally used in peptidechemistry, such as, for example, carbodiimides, phosphonium reagents,uronium reagents, acid halides or activated esters, to give a compoundof the formula IV ##STR5## in which PG, X, B' and R¹ are as definedabove

and subsequently converting this compound to a compound of the formula Iby eliminating the ester protective group R¹ under weakly alkalineconditions using alkali metal hydroxide solution, such as, for example,NaOH, LiOH, KOH, or by tertiary amine compounds in water, such as, forexample, triethylamine, or else enzymatically with the aid of esterasesor lipases at 0-50° C., preferably at room temperature, in a suitablesolvent, such as dioxane, water, tetrahydrofuran, methanol, water ormixtures of the solvents.

Activation methods conventionally used in peptide synthesis aredescribed, for example, in Houben-Weyl, Methoden der organischen Chemie[Methods in Organic Chemistry], Volume 15/2, Georg Thieme VerlagStuttgart 1974, or further reagents are described in the particularreferences, for example BOP (B. Castro, J. R. Dormoy, G. Evin and C.Selve, Tetrahedron Lett. 1975, 1219-1222), PyBOP (J. Coste, D. Le-Nguyenand B. Castro, Tetrahedron Lett. 1990, 205-208), BroP (J. Coste, M.-N.Dufour, A. Pantaloni and B. Castro, Tetrahedron Lett. 1990, 669-672),PyBroP (J. Coste, E. Frerot, P. Jouin and B. Castro, Tetrahedron Lett.1991, 1967-1970) and uronium reagents, such as, for example, HBTU (V.Dourtoglou, B. Gross, V. Lambropoulou, C. Zioudrou, Synthesis 1984,572-574), TBTU, TPTU, TSTU, TNTU, (R. Knorr, A. Trzeciak, W. Bannwarthand D. Gillessen, Tetrahedron Letters 1989, 1927-1930), TOTU (EP-A-0 460446), HATU (L. A. Carpino, J. Am. Chem. Soc. 1993, 115, 4397-4398),HAPyU, TaPipU (A. Ehrlich, S. Rothemund, M. Brudel, M. Beyermann, L. A.Carpino and M. Bienert, Tetrahedron Lett. 1993, 4781-4784), BOI (K.Akaji, N. Kuriyama, T. Kimura, Y. Fujiwara and Y. Kiso, TetrahedronLett. 1992, 3177-3180) or acid chlorides or acid fluorides (L. A.Carpino, H. G. Chao, M. Beyermann and M. Bienert, J. Org. Chem.,56(1991), 2635; J.-N. Bertho, A. Loffet, C. Pinel, F. Reuther and G.Sennyey in E. Giralt and D. Andreu (Eds.) Peptides 1990, Escom SciencePublishers B.V. 1991, pp. 53-54; J. Green and K. Bradley, Tetrahedron1993, 4141-4146), 2,4,6-mesitylenesulfonyl-3-nitro-1,2,4-triazolide(MSNT) (B. Blankemeyer-Menge, M. Nimitz and R. Frank, Tetrahedron Lett.1990, 1701-1704), 2,5-diphenyl-2,3-dihydro-3-oxo-4-hydroxythiophenedioxide (TDO) (R. Kirstgen, R. C. Sheppard, W. Steglich, J. Chem. Soc.Chem. Commun. 1987, 1870-1871) or activated esters (D. Hudson) PeptideRes. 1990, 51-55).

Preferred is the use of carbodiimides, for exampledicyclohexylcarbodiimide or diisopropylcarbodiimide. Other reagentswhich are preferably used are phosphonium reagents, such as, forexample, PyBOP or PbBroP, uronium reagents, such as, for example HBTU,TBTU, TPTU, TSTU, TNTU, TOTU or HATU, BOI or acid chlorides or acidfluorides.

To synthesize the compounds of the formula II, aminoethylglycine,hydroxyethylglycine, mercaptoethylglycine or their corresponding estersare provided with the corresponding protective group which is labile toweak acids. The protective group which is labile to weak acids isintroduced with the aid of processes per se known from the literature,some of which have been modified. Examples of suitable reagents aret-Bumeoc fluoride, Adpoc azide, Bpoc azide, Ddz (phenyl)carbonate, TrtCl, Mtt Cl, Mmt Cl, Mmt Cl, Dmt Cl, Pixyl Cl. In this reaction, thesolubility of the aminoethylglycine can be improved while simultaneouslyprotecting the acid function by reacting it with customary silylationreagents, such as, for example, bis-trimethylsilylacetamide. After thereaction with the protective group reagents, this temporary protectivegroup is eliminated by adding water or alcohols to the reaction mixture.The aminoethylglycine or the corresponding aminoethylglycine ester usedas the starting material are prepared by a method known from theliterature (E. P. Heimer, H. E. Gallo-Torres, A. M. Felix, M. Ahmad, T.J. Lambros, F. Scheidl and J. Meienhofer, Int. J. Peptide Protein Res.23, 1984, 203-211) 2-aminoethylglycine (H-Aeg-OH).

A further process for the preparation of aminoethylglycine consists insubjecting glyoxylic acid to reductive amination with ethylene diamineand is described in the application titled "Process for the preparationof aminoethylglycine" (HOE 94/F 061, DE-P 44 08 530.3) filedsimultaneously.

2-Hydroxyethylglycine or 2-mercaptoethylglycine is synthesized, forexample, by subjecting glyoxylic acid or glyoxylic esters to reductiveamination with aminoethanol or cysteamine using hydrogen onpalladium-on-charcoal or, in the case of 2-mercaptoethylglycine,preferably using sodium cyanoborohydride or sodium triacetoxyborohydrideas the reducing agent.

The compound of the formula I, which is derived from2-mercaptoethylglycine, can also be obtained by first reacting2-mercaptoethylamine or its hydrochloride with the correspondingtriphenyl derivative, such as, for example, its halides or alcohols, inacetic acid or mixtures of acetic acid/water. Under the acidicconditions, the reaction is preferably effected on the sulfur of2-mercaptoethylamine via the corresponding trityl cation. The use of(4-methoxyphenyl)diphenylmethyl chloride is preferred for introducingthe Mmt group, and the use of di-(4-methoxyphenyl)phenylmethyl chloridefor introducing the Dmt group. The amino group is subsequently alkylatedby reaction with haloacetic esters, preferably bromoacetic esters, usingan organic auxiliary base, such as, for example, diisopropyl ethylamine, in a suitable solvent, such as, for example, DMF. The resultingcompound of the formula II in which PG is a trityl-type protectivegroup, X is S and R¹ is an ester protective group can then be reacted inthe manner described above to give the corresponding compounds of theformula I.

The acetic acid derivatives of the nucleobases, of the formula III, canbe retained by alkylating the corresponding nucleobases or thenucleobases which are protected in the exocyclic hydroxyl or aminofunction using chloroacetic acid, bromoacetic acid, iodoacetic acid, ortheir esters. For preference, temporary protective groups areadditionally introduced on the nucleobase for the purposes of selectivealkylation. Protective groups which are suitable for the nucleobases areall protective groups which are compatible with the protective group PGwhich is labile to weak acids. Protective groups which are preferablyused for the exocyclic amino function are, for example, the benzoyl,isobutanoyl, acetyl, phenoxyacetyl, 4-(t-butyl)benzoyl,4-(t-butyl)phenoxyacetyl, 4-(methoxy)benzoyl,2-(4-nitrophenyl)ethyloxycarbonyl,2-(2,4-dinitrophenyl)ethyloxycarbonyl, 9-fluorenylmethoxycarbonyl,diphenylcarbamoyl, or formamidine group.

Particularly preferred are the benzoyl, isobutanoyl, 4-(t-butyl)benzoyl,2-(4-nitrophenyl)ethyloxycarbonyl,2-(2,4-dinitrophenyl)ethyloxycarbonyl, 9-fluorenylmethoxycarbonyl,4-(methoxy)benzoyl or para-(t-butyl)phenoxyacetyl orpara-nitrophenyl-2-ethyloxycarbonyl group and, in the case of guanine, acombination of the 2-N-acetyl with the 6-O-diphenylcarbamoyl group.

An alternative process for the preparation of the compounds of theformula I in which Y is CH₂ consists in reacting

a compound of the formula II ##STR6## in which PG and X are as definedabove and

R¹ is hydrogen or a temporary silyl protective group, such as, forexample, trimethylsilyl,

with a compound of the formula V

    B'--CH.sub.2 --CO--R.sup.2                                 (V),

in which

B' is as defined above and

R² is halogen, such as, for example, fluorine, chlorine or bromine, orthe radical of an active ester, such as, for example, OBt, OObt, OPfp,ONSu,

at 0-40° C., preferably 20-30° C., in a suitable solvent, such as, forexample, DMF, NMP, acetonitrile, dichloromethane or mixtures of thesesolvents, it optionally being possible to protect the acid function inthe compound of the formula II temporarily by reacting it with customarysilylation reagents, such as, for example, bis-trimethylsilylacetamide,and to eliminate these temporarily protective groups--after the reactionwith the compound of the formula V--by adding water or alcohols to thereaction mixture.

A further alternative process for the preparation of the compounds ofthe formula I where Y is CH₂ consists in reacting

a compound of the formula II ##STR7## in which PG and X are as definedabove and

R¹ is an ester protective group, such as, for example, methyl, ethyl,butyl, 2-(methoxyethoxy)ethyl and the like,

with a haloacetic acid derivative, such as, for example, chloroacetylchloride, bromoacetyl bromide, bromoacetyl chloride or iodoacetylchloride, in a suitable solvent, such as, for example, tetrahydrofuran,dichloromethane or DMF, using an auxiliary base, such as, for example,triethylamine, N-ethylmorpholine or diisopropylethylamine, to give thecompound of the formula VI ##STR8## in which Hal is Cl, Br or I,preferably Br or Cl, very particularly Cl, and

PG, X and R¹ are as defined above,

reacting this intermediate of the formula VI with the optionallyprotected nucleobase B' and an auxiliary base, for example potassiumcarbonate, in a suitable solvent, for example DMF or NMP, to give thecompound of the formula IV ##STR9## and subsequently converting thiscompound to a compound of the formula I by eliminating the esterprotective group R¹ using alkali metal hydroxide solution, such as, forexample, NaOH, LiOH or KOH, or else enzymatically with the aid ofesterases or lipases at 0-50° C., preferably at room temperature, in asuitable solvent, such as dioxane, water, tetrahydrofuran, methanol,water or mixtures of these solvents.

Compounds of the formula I in which Y is O or NH are obtained byreacting

a compound of the formula II ##STR10## in which PG and X are as definedabove and

R¹ is an ester protective group, such as, for example, methyl, ethyl,butyl or 2-(methoxyethoxy)ethyl, or a temporary ester protective group,such as, for example, trimethylsilyl,

with a compound of the formula VII

    B'--Y--CO--R.sup.3                                         (VII)

in which

B' is as defined above,

Y is O or NH and

R³ is Cl, ONp or ONSu

at 0-40° C., preferably 20-30° C., in a suitable solvent, such as DMF,acetonitrile or dichloromethane, or mixtures of these solvents, andsubsequently eliminating the ester protective group R¹ using alkalimetal hydroxide solution, such as, for example, NaOH, LiOH or KOH, orelse enzymatically with the aid of esterases or lipases at 0-50° C.,preferably at room temperature, in a suitable solvent, such as dioxane,water, tetrahydrofuran, methanol, water or mixtures of these solvents.

The N-amino- or N-hydroxy-nucleobases required as starting material forthis latter reaction are obtained by known processes, such as, forexample, as described by K. Kohda, I. Kobayashi, K. Itano, S. Asano, Y.Kawazoe (Tetrahedron 49, 3947-3958 (1993)) and then reacted withphosgene or chloroformic esters to give the carbamates or carbonates ofthe formula VII.

The abbreviations used for amino acids correspond to the three-lettercode conventionally used in peptide chemistry, as it is described inEurop. J. Biochem. 138, 9 (1984). Other abbreviations used are listedhereinbelow.

    ______________________________________                                        Aeg       N-(2-aminoethyl)glycyl,                                                --NH--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CO--                              Aeg (A.sup.MeOBz) N-(2-aminoethyl)-N-((9-N.sup.6 -4-methoxybenzoyl)-                    adenosyl)acetyl)glycyl                                              Aeg (C.sup.Bz) N-(2-aminoethyl)-N-((1-(N.sup.4 -benzoyl)-cytosyl)-                      acetyl)glycyl                                                       Aeg (C.sup.MeOBz) N-(2-aminoethyl)-N-((1-(N.sup.4 -4-methoxybenzoyl)-                   cytosyl)acetyl)glycyl                                               Aeg (C.sup.tBuBz) N-(2-aminoethyl)-N-((1-(N.sup.4 -4-tert-butylbenzoyl)-       cytosyl)acetyl)glycyl                                                        Aeg (G.sup.iBu) N-(2-aminoethyl)-N-((9-N.sup.2 -isobutanoyl)guanosyl)-                  glycyl                                                              Aeg (G.sup.2-Ac, 4-Dpc) N-(2-aminoethyl)-N-((9-(N.sup.2 -acetyl-O.sup.4               -diphenylcar-                                                          bamoyl)guanosyl)glycyl                                                       Aeg (Im) N-(2-amino)ethyl-N-((1-imidazolyl)acetyl)glycyl                      Aeg (Im.sup.4-Nitro) N-(2-aminoethyl)-N-((1-(4-nitro)imidazolyl)acetyl)-       glycyl                                                                       Aeg (T) N-(2-aminoethyl)-N-((1-thyminyl)acetyl)glycyl                         Aeg (Triaz) N-(2-aminoethyl)-N-((1-(1,2,4)triazolyl)acetyl)-                   glycyl                                                                       Bnpeoc 2,2-[bis(4-nitrophenyl)]ethoxycarbonyl)                                Boc tert-butyloxycarbonyl                                                     BOI 2-(benzotriazol-1-yl)oxy-1,3-dimethylimidazolidinium                       hexafluorophosphate                                                          BOP benzotriazolyl-1-oxy-tris(dimethylamino)-                                  phosphonium hexafluorophosphate                                              BroP bromotris(dimethylamino)phosphonium hexafluoro-                           phosphate                                                                    BSA N,O-bis-(trimethylsilyl)acetamide                                         But tert-butyl                                                                Bz benzoyl                                                                    Bzl benzyl                                                                    Cl-Z 4-chlorobenzyloxycarbonyl                                                CPG controlled pore glass                                                     DBU 1,8-diazabicyclo[5.4.0]undec-7-ene                                        DCM dichloromethane                                                           Ddz 3,5-dimethoxyphenyl-2-propyl-2-oxycarbonyl                                DMF dimethylformamide                                                         Dmt di-(4-methoxyphenyl)phenylmethyl                                          Dnpeoc 2-(2,4-dinitrophenyl)ethoxycarbonyl                                    Dpc diphenylcarbamoyl                                                         FAM flourescein radical                                                       Fm 9-fluorenylmethyl                                                          Fmoc 9-fluorenylmethyloxycarbonyl                                             H-Aeg-OH N-(2-aminoethyl)glycine                                              HAPyU O-(7-Azabenzotriazol-1-yl)-1,1,3,3-bis(tetramethylene)                   uronium hexafluorophosphate                                                  HATU O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium                     hexafluorophosphate                                                          HBTU O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluronium                          hexafluorophosphate                                                          HOBt 1-hydroxybenzotriazole                                                   HONSu N-hydroxysuccinimide                                                    HOObt 3-hydroxy-4-oxo-3,4-dihydrobenzotriazine                                iBu isobutanoyl                                                               MeOBz 4-methoxybenzoyl                                                        Mmt 4-methoxytriphenylmethyl                                                  Moz 4-methoxybenzyloxycarbonyl                                                MSNT 2,4,6-mesitylenesulfonyl-3-nitro-1,2,4-triazolide                        Mtt 4-methylphenyl)diphenylmethyl                                             NBA nitrobenzyl alcohol                                                       NMP N-methylpyrrolidine                                                       Oeg N-(2-oxyethyl)glycyl,                                                      --O--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CO--                               Pixyl 9-(9-phenyl)xanthenyl                                                   PyBOP benzotriazolyl-1-oxy-tripyrrolidinophosphonium                           hexafluorophosphate                                                          PyBroP bromotripyrrolidinophosphonium hexafluorophosphate                     TAPipU O-(7-azabenzotriazol-1-yl)-1,1,3,3-bis(penta-                           methylene)uronium tetrafluoroborate                                          TBTU O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluronium                          tetrafluoroborate                                                            tBu tert-butyl                                                                tBuBz 4-tert-butylbenzoyl                                                     TDBTU O-(3,4-dihydro-4-oxo-1,2,3-benzotriazin-3-yl)-                           1,1,3,3-tetramethyluronium tetrafluoroborate                                 TDO 2,5-diphenyl-2,3-dihydro-3-oxo-4-hydroxythiophene-                         dioxide                                                                      Teg N-(2-thioethyl)glycyl,                                                     --S--CH.sub.2 CH.sub.2 --NH--CH.sub.2 --CO--                                 TFA trifluoroacetic acid                                                      THF tetrahydrofuran                                                           TNTU O-[(5-norbonene-2,3-dicarboximido]-1,1,3,3-tetra-                         methyluronium tetrafluoroborate                                              TOTU O-[(cyano(ethoxycarbonyl)methylene)amino]-1,1,3,3-                        tetramethyluronium tetrafluoroborate                                         TPTU O-(1,2-dihydro-2-oxo-1-pyridyl)-1,1,3,3'-tetramethyl-                     uronium tetrafluoroborate                                                    Trt trityl                                                                    TSTU O-(N-succinimidyl)-1,1,3,3-tetramethyluroniumtetra-                       fluoroborate                                                                 Z benzyloxycarbonyl                                                           MS (ES.sup.+) electrostatic spraying mass spectrum (positive ion)                      MS (ES.sup.-) electrostatic spraying mass spectrum (negative                 ion)                                                                  MS (DCI) desorption chemical ionization mass spectrum                         MS (FAB) Fast Atom Bombardment mass spectrum                                ______________________________________                                    

EXAMPLES Example 1 Preparation of N-(hydroxyethyl)glycine

H-Oeg-OH

46 g of glyoxylic acid monohydrate are dissolved in 1,000 ml of water,and 30.2 ml of 2-aminoethanol are then added with stirring and cooling.The mixture is treated with 10 g of catalyst (10% Pd/C) and hydrogenatedin the autoclave at room temperature and a hydrogen pressure of 10 bar.For working up, the catalyst is filtered off with suction and thefiltrate is concentrated to dryness in vacuo. The residue is subjectedto two distillations using a small amount of toluene. This crude productis digested using 250 ml of hot methanol, filtered off with suctionwhile warm, washed with a small amount of methanol and then dried in adesiccator.

Yield: 49.56 g.

M.p.: 178-180° C., decomposition.

R_(f) =0.36 (n-butanol/acetic acid/water/ethyl acetate 1:1:1:1).

MS(DCl): 120 (M+H)⁺.

Example 2 Synthesis ofN-(9-fluorenylmethoxycarbonyl)-N-(hydroxyethyl)glycine

H-Oeg(Fmoc)-OH

10.08 g of sodium hydrogen carbonate are dissolved in 150 ml of water,and 7.15 g of N-(hydroxyethyl)glycine are then added with stirring.After approximately 5 minutes, a clear solution is obtained, and 20.22 gof Fmoc-ONSu in 300 ml of dioxane are added dropwise with vigorousstirring. Stirring is continued for 3 hours at room temperature and themixture is then allowed to stand overnight at 4° C. The solution isfiltered and the filtrate is concentrated in vacuo on a rotaryevaporator. The residue is taken up in 100 ml of water and the pH isbrought to 2 by adding portions of potassium hydrogen sulfate solution.The mixture is then extracted three times using in each case 150 ml ofethyl acetate, the organic phase is washed four times using in each case50 ml of water and dried over sodium sulfate. The desiccant is filteredoff, and the filtrate is concentrated to dryness in vacuo on a rotaryevaporator. The residue is treated with 200 ml of diisopropyl ether andtriturated, which results in crystallization. The mixture is allowed tostand overnight, and the precipitate is filtered off with suction andrecrystallized from ethyl acetate/diisopropyl ether. The resultingproduct is filtered off with suction and dried in a desiccator.

Yield: 15.8 g.

M.p.: 114-116° C., decomposition.

R_(f) =0.62 (n-butanol/acetic acid/water 3:1:1).

MS(ES⁺): 342 (M+H)⁺.

Example 3 Synthesis ofN-(9-fluorenylmethoxycarbonyl)-N-(4-methoxyphenyldiphenylmethoxyethyl)glycine

Mmt-Oeg(Fmoc)-OH

14.74 g of N-(fluorenylmethoxycarbonyl)-N-(hydroxyethyl)glycine aredissolved in 160 ml of pyridine, and 13.31 g of4-methoxyphenyldiphenylmethyl chloride are added. The mixture is stirredfor 2 hours at room temperature and allowed to stand overnight at 4° C.The solvent is then stripped off in vacuo on a rotary evaporator, theresidue is taken up in 300 ml of ethyl acetate, and the mixture isextracted three times using in each case 30 ml of saturated aqueousbicarbonate solution. The extracts are then also washed three timesusing in each case 30 ml of water, and the organic phase is dried oversodium sulfate. The desiccant is filtered off, and the filtrate isconcentrated to a volume of approximately 70 ml and added dropwise withstirring to 700 ml of diisopropyl ether. The product which hasprecipitated is filtered off with suction and dried in a desiccator.

Yield: 19.95 g.

R_(f) =0.45 (ethyl acetate/methanol/triethylamine=60:40:1).

MS(FAB/MeOH/NBA): 658.3 (M+2Na)⁺.

Example 4 N⁴ -(4-Methoxybenzoyl)cytosine Example 4a

Cytosine (11.1 g) is suspended in dry pyridine (250 ml). Using asyringe, 4-methoxybenzoyl chloride (17.06 g) is then added dropwise. Avirtually clear solution is formed from which a precipitate separatesout after approximately 10 minutes. Stirring is continued forapproximately 1.5 hours at room temperature, and the precipitate is thenfiltered off. The precipitate is the desired product.

Yield: 20.5 g.

MS(ES⁺): 246 (M+H)⁺.

R_(f) =0.77 (dichloromethane:isopropanol/8:2).

Example 4b

Cytosine (2.8 g) is suspended in dry DMF (100 ml) and treated withtriethylamine (2 ml). Using a syringe, 4-methoxybenzoic anhydride (7.2g) is then added dropwise. A virtually clear solution is formed fromwhich a precipitate separates out after some time. Stirring is continuedfor approximately 2 hours at room temperature, and the precipitate isthen filtered off. The precipitate is the desired product.

Yield: 5.9 g.

MS(ES⁺): 246 (M+H)⁺.

R_(f) =0.77 (dichloromethane:isopropanol/8:2).

Example 5 N⁴ -(4-Methoxybenzoyl)-N'-methoxycarbonylmethylcystosine

N⁴ -(4-Methoxybenzoyl)cytosine (12.25 g) is suspended in dry DMF (250ml), and sodium hydride (1.25 g) is added in portions. The mixture isheated briefly at 60° C. until the evolution of hydrogen has ceased.Methyl bromoacetate (4.75 ml) is subsequently added dropwise at roomtemperature using a syringe. Stirring is continued for 1 hour at roomtemperature, and the mixture is then treated with a small amount ofcarbon dioxide in methanol. The solvent is stripped off in vacuo, andthe residue which remains is treated with dichloromethane and thenfiltered. The filter residue is also washed with a small amount of waterand then dried in vacuo.

Yield 12.86 g.

M.p.: 219° C.

MS(ES⁺): 318 (M+H)⁺.

R_(f) =0.71 (n-butanol:acetic acid:water/3:1:1).

Example 6 N⁴ (4-Methoxybenzoyl)-N¹ -carboxymethylcytosine

N⁴ -(4-Methoxybenzoyl)-N¹ -methoxycarbonylmethylcytosine (12.5 g) issuspended in water (100 ml) and 2N aqueous sodium hydroxide solution isadded dropwise at 0° C. while the pH is checked (pH 12) until the methylester is hydrolyzed. The course of the reaction is monitored by means ofTLC. The reaction solution is then filtered, and the pH of the filtrateis brought to 3 using 2M potassium hydrogen sulfate solution, duringwhich process the product precipitates. The precipitate is filtered off,washed with a small amount of cold water and dried in vacuo.

Yield: 11.5 g.

M.p.: 270-275° C., decomposition.

MS(ES⁺): 304 (M+H)⁺.

R_(f) =0.53 (n-butanol:acetic acid:water/3:1:1).

Example 7 N⁴ -(4-tert-Butylbenzoyl)cytosine

Cytosine (11.1 g) is suspended in dry DMF (250 ml), and triethylamine(15.4 ml) is added. 4-tert-Butylbenzoyl chloride (18.6 ml) is then addeddropwise using a syringe. Stirring is then continued for approximately 4hours at room temperature and more 4-tert-butylbenzoyl chloride (3.7 ml)is then added. After a further 2 hours, the reaction is quenched byadding a small amount of methanol. The solvent is removed in vacuo, andthe residue is treated with dichloromethane and water. The solid whichprecipitates in this process is the desired substance. The precipitateis filtered off and dried in vacuo.

Yield: 15.7 g.

MS(DCI): 272 (M+H)⁺.

R_(f) =0.55 (dichloromethane:methanol/9:1).

Example 8 N⁴ -(4-tert-Butylbenzoyl)-N¹ -methoxycarbonylmethylcytosine

N⁴ -(4-tert-Butylbenzoyl)cytosine (14.96 g) is suspended in dry DMF (250ml), sodium hydride (1.32 g) is added in portions, and the mixture isstirred for 1 hour at room temperature until the evolution of hydrogenhas ceased. Methyl bromoacetate (5.2 ml) is subsequently added dropwiseat room temperature using a syringe. Stirring is continued for 1 hour atroom temperature, and methanol (10 ml) is then added. The solvent isstripped off in vacuo, and the residue which remains is partitionedbetween dichloromethane and water. The organic phase is washed withwater, dried over sodium sulfate, filtered and then concentrated invacuo. The resulting crude product is recrystallized from isopropanoland then dried in vacuo.

Yield: 8.0 g.

M.p.: 189-193° C., decomposition.

MS(DCI): 344 (M+H)⁺.

R_(f) =0.72 (dichloromethane:methanol/9:1).

Example 9 N⁴ -(4-tert-Butylbenzoyl)-N¹ -carboxymethylcytosine

N⁴ -(4-tert-Butylbenzoyl)-N¹ -methoxycarbonylmethylcytosine (8.0 g) isdissolved in a mixture of dioxane (50 ml) and water (10 ml), and 2Naqueous sodium hydroxide solution is added dropwise with stirring atroom temperature (pH 11-12). When hydrolysis of the methyl ester iscomplete--TLC check--, the pH of the reaction solution is brought to 3using 2M potassium hydrogen sulfate solution. The precipitate which hasseparated out is filtered off with suction. This crude product isdissolved in sodium carbonate solution and reprecipitated by adding 2Mpotassium hydrogen sulfate solution. The product is filtered off withsuction, washed with a small amount of water and dried in vacuo.

Yield: 6.62 g.

M.p.: from 285° C., decomposition.

MS(DCI): 330 (M+H)⁺.

R_(f) =0.2 (dichloromethane:methanol/9:1).

Example 10 N⁴ -(Benzoyl)cytosine

Cytosine (5.55 g) is suspended in dry pyridine (100 ml). Using asyringe, benzoyl chloride (6.4 ml) is then added dropwise. A virtuallyclear solution is formed from which a precipitate separates out afterapproximately 5 minutes. Stirring is continued for approximately 2 hoursat room temperature, methanol (100 ml) is then added, and the mixture isconcentrated in vacuo. The residue is stirred with isopropanol,filtered, washed with a small amount of methanol and diethyl ether, anddried in vacuo.

Yield: 10.1 g.

M.p.: <305° C.

MS(DCI): 216 (M+H)⁺.

Example 11 N⁴ -(Benzoyl)-N¹ -methoxycarbonylmethylcytosine

N⁴ -(Benzoyl)cytosine (4.3 g) is suspended in dry DMF (60 ml), andsodium hydride (0.48 g) is added in portions). The mixture is heatedbriefly at 40° C. until the evolution of hydrogen has ceased. Methylbromoacetate (2.1 ml) is subsequently added dropwise at roomtemperature, using a syringe. Stirring is continued for 2.5 hours atroom temperature, and the mixture is then treated with methanol (10 ml).The solvent is stripped off in vacuo, and the residue which remains istreated with dichloromethane and then filtered. The filter residue isalso washed with a small amount of water and then dried in vacuo.

Yield: 6.45 g.

M.p.: 234° C.

MS(DCI): 288 (M+H)⁺.

R_(f) =0.85 (ethyl acetate:methanol/8:2).

Example 12 N⁴ -(Benzoyl)-N¹ -carboxymethylcytosine

N⁴ -(Benzoyl)-N 1-methoxycarbonylmethylcytosine (5.85 g) is dissolved ina mixture of dioxane (80 ml) and water (40 ml) and 2N aqueous sodiumhydroxide solution (11.25 ml) is added dropwise at room temperature,with stirring (pH 11-12). When hydrolysis of the methyl ester iscomplete--TLC check--, the pH of the reaction solution is brought to 3using 2M potassium hydrogen sulfate solution. The precipitate which hasseparated out is filtered off with suction. This crude product isdissolved in sodium carbonate solution and reprecipitated by adding 2Mpotassium hydrogen sulfate solution. The product is filtered off withsuction, washed with a small amount of water and dried in vacuo.

Yield: 5.84 g.

M.p.: from 283-286° C., decomposition.

MS(ES⁺): 274 (M+H)⁺.

R_(f) =0.15 (ethyl acetate:methanol/8:2).

Example 13 N⁴ -(tert-Butyloxycarbonyl)cytosine

Cytosine (11.1 g) is suspended in dry pyridine (250 ml). Di-tert-butyldicarbonate (21.8 g) and 1 spatula-tipful of 4-dimethylaminopyridine arethen added. The mixture is stirred for 6 hours at 60° C., during whichprocess a dense precipitate is formed. The reaction solution is cooled,and the precipitate is filtered off with suction. The precipitate isstirred with water under hot conditions, filtered off with suction anddried in vacuo.

Yield: 10.26 g.

MS(DCI): 212 (M+H)⁺.

R_(f) =0.6 (dichloromethane:methanol/6:4).

Example 14 N⁴ -(tert-Butyloxycarbonyl)-N¹ -methoxycarbonylmethylcytosine

N⁴ -(tert-Butyloxycarbonyl)cytosine (1.6 g) is suspended in dry DMF (30ml), sodium hydride (0.19 g) is added in portions, and the mixture isstirred for 1.5 hours at room temperature until the evolution ofhydrogen has ceased. Methyl bromoacetate (0.84 ml) is subsequently addeddropwise at room temperature using a syringe. stirring is continued for5 hours at room temperature, and the mixture is then treated withmethanol (1 ml). The solvent is stripped off in vacuo, and the residuewhich remains is purified by column chromatography on silica gel usingdichloromethane as the eluent. The product-containing fractions arecombined and concentrated in vacuo.

Yield: 1.1 g.

MS(DCI): 284 (M+H)⁺.

R_(f) =0.5 (dichloromethane:methanol/95:5).

Example 15 N⁴ -(tert-Butyloxycarbonyl)-N¹ -carboxymethylcytosine

N⁴ -(tert-Butyloxycarbonyl)-N¹ -methoxycarbonylmethylcytosine (4.2 g) issuspended in water (30 ml) and 2N aqueous sodium hydroxide solution isadded dropwise at 0° C. while the pH is checked (pH 12) until the methylester is hydrolyzed. The course of the reaction is monitored by means ofTLC. The pH of the reaction solution is then brought to 3 using aceticacid, and the solvent is distilled off in vacuo. The crude product ispurified by column chromatography on silica gel usingdichloromethane:methanol:triethylamine/8:1:1 as the eluent. Theproduct-containing fractions are combined and concentrated in vacuo.

Yield: 3.5 g.

M.p.: 95-98° C., decomposition.

MS(FAB/NBA): 270.2 (M+H)⁺.

R_(f) =0.35 (dichloromethane:methanol:triethylamine/8:1:1).

Example 16 N⁶ -(4-Methoxybenzoyl)adenine

Adenine (13.5 g) is suspended in dry pyridine (250 ml). 4-Methoxybenzoylchloride (17.06 g) is then added drop-wise using a syringe. The mixtureis stirred for 3 hours at 100° C. and allowed to stand overnight at roomtemperature. The reaction solution is then treated with methanol, andthe solvent is subsequently distilled off in vacuo. The residue iscoevaporated two more times using a small amount of toluene and thenstirred with hot isopropanol. The mixture is allowed to cool slowly, andthe product which has precipitated is filtered off with suction anddried in vacuo.

Yield: 22.2 g.

M.p.: 212-214° C.

MS(ES⁺): 270 (M+H)⁺.

R_(f) =0.67 (dichloromethane:isopropanol/8:2).

Example 17 N⁶ -(4-Methoxybenzoyl)-N⁹ -methyloxycarbonylmethyladenine

N⁶ -(4-Methoxybenzoyl)adenine (8.01 g) is suspended in dry DMF (150 ml),sodium hydride (0.75 g) is added in portions, and the mixture is stirredat room temperature for 0.5 hours until the evolution of hydrogen hasceased. Methyl bromoacetate (2.85 ml) is subsequently added dropwise atroom temperature using a syringe. Stirring is continued for 2 hours atroom temperature, and the mixture is then treated with a small amount ofcarbon dioxide in methanol. The solvent is stripped off in vacuo, andthe residue which remains is treated with dichloromethane/water and thenfiltered. The filter residue is also washed with a small amount of waterand then dried in vacuo.

Yield: 4.03 g.

MS(ES⁺): 342 (M+H)⁺.

R_(f) =0.76 (ethyl acetate:methanol/8:2).

Example 18 N⁶ -(4-Methoxybenzoyl)-N⁹ -carboxymethyladenine

N⁶ -(4-Methoxybenzoyl)-N⁹ -methyloxycarbonylmethyladenine (1.71 g) issuspended in water (40 ml), and 2N aqueous sodium hydroxide solution isadded dropwise at 0° C. while the pH is checked (pH 11.2) until themethyl ester is hydrolyzed. The course of the reaction is monitored bymeans of TLC. The reaction solution is then filtered, and the pH of thefiltrate is brought to 3 using 2M potassium hydrogen sulfate solution,during which process the product precipitates. The precipitate isfiltered off, washed with a small amount of cold water and dried invacuo.

Yield: 1.52 g.

M.p.: 222-223° C., decomposition.

MS(ES⁺): 328 (M+H)⁺.

R_(f) =0.2 (n-butanol/acetic acid/water 3:1:1).

Example 19 N² -(Isobutanoyl)guanine

Guanine (3.02 g) is suspended in dry DMF (40 ml), and triethylamine(1.45 ml) is added. Isobutyryl chloride (2.12 g) is then added dropwiseusing a syringe. The mixture is stirred for 3 hours at 100° C., thereaction solution is then treated with methanol, and the solvent issubsequently distilled off in vacuo. The residue is stirred with hotisopropanol, and the product which has precipitated is filtered off withsuction and dried in vacuo.

Yield: 2.67 g.

R_(f) =0.45 (n-butanol/acetic acid/water 3:1:1).

Example 20 N² -(Isobutanoyl)-N⁹ -methyloxycarbonylmethylguanine

N-(Isobutanoyl)guanine (4.42 g) is suspended in dry DMF (50 ml), sodiumhydride (0.5 g) is added in portions, and the mixture is stirred for 1hour at room temperature until the evolution of hydrogen has ceased.Methyl bromoacetate (1.9 ml) is subsequently added dropwise at roomtemperature using a syringe. Stirring is continued for 1 hour at roomtemperature and the mixture is then treated with a small amount ofcarbon dioxide in methanol. The solvent is stripped of f in vacuo, andthe residue which remains is purified by means of column chromatographyon silica gel using dichloromethane:methanol/95:5 as the eluent. Theproduct-containing fractions are combined and concentrated in vacuo.

Yield: 2.35 g.

MS(DCl): 294(M+H)⁺.

R_(f) =0.58 (dichloromethane:methanol/9:1).

Example 21 N² -(Isobutanoyl)-N⁹ -carboxymethylguanine

N² -(Isobutanoyl)-N⁹ -methyloxycarbonylmethylguanin e (9.68 g) issuspended in water (100 ml), and 2N aqueous sodium hydroxide solution isadded dropwise at 0° C. while the pH is checked (pH 11) until the methylester is hydrolyzed. The course of the reaction is monitored by means ofTLC. The reaction solution is then filtered, the pH of the filtrate isbrought to 3 using 2M potassium hydrogen sulfate solution, and themixture is extracted using ethyl acetate. The organic phase isdiscarded. The aqueous phase is concentrated in vacuo to a volume ofapproximately 20 ml and covered with a layer of ethyl acetate. Theprecipitate which separates out slowly is filtered off with suction anddried.

Yield: 1.35 g.

MS(ES⁺): 342(M+H)⁺.

R_(f) =0.34 (n-Butanol:acetic acid:water/3:1:1).

Example 22 Preparation ofN-(4-methoxyphenyldiphenylmethylaminoethyl)glycine methyl ester

Mmt-Aeg-OMe

14.76 g of N-(aminoethyl)glycine methyl ester dihydrochloride (H-Aeg-OMe2 HCl) are suspended in 300 ml of DMF and the suspension is treated with30.2 ml of triethylamine, with stirring. The mixture is cooled toapproximately 4° C., and 22.23 g of Mmt-Cl, dissolved in 100 ml ofdichloromethane, are slowly added dropwise, with vigorous stirring.Stirring is continued for 2.5 hours at room temperature. Precipitatedtriethylamine hydrochloride is then filtered off, 10 ml of ethanol areadded to the solution, and the filtrate is concentrated in vacuo. Theresidue is then chromatographed on silica gel using a mixture of diethylether/petroleum ether/triethylamine 200:100:3. The product-containingfractions are combined and evaporated to dryness in vacuo.

Yield: 18.82 g of oil which crystallizes slowly upon standing.

R_(f) =0.16 (diethyl ether/petroleum ether 2:1 together with 1% oftriethylamine).

MS(FAB/MeOH/NBA/LiCl): 411.2 (M+Li)⁺.

Example 23 Synthesis protocol for the preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-thyminyl)acetyl)glycin

(Mmt-Aeg(T)-OH)

1.21 g (3 mmol) of Mmt-Aeg-OMe are dissolved in 20 ml of THF, and 0.24ml of chloroacetyl chloride, dissolved in 10 ml of THF, and 0.42 ml oftriethylamine, dissolved in 10 ml of THF, are simultaneously slowlyadded dropwise with cooling and vigorous stirring. The mixture is thenallowed to afterreact for 30 minutes, triethylamine hydrochloride whichhas precipitated is filtered off with suction, and the solution istreated with 30 ml of dry DMF. The THF is subsequently stripped off invacuo on a rotary evaporator, and 0.756 g (6 mmol) of thymine and 1.66 g(12 mmol) of finely pulverulent potassium carbonate are added insuccession to the solution of Mmt-Aeg(chloroacetyl)-OMe in DMF. Thismixture is stirred for 48 hours at room temperature, and undissolvedmatter is then filtered off with suction. The solvent is stripped off,and the residue is partitioned between water and ethyl acetate. Afterthe solvent has been stripped off, the crude product which remains inthe ethyl acetate phase is hydrolyzed in a mixture ofdioxane/methanol/water by adding 35 ml of 0.2 N NaOH. The residue whichis obtained after the solvent has been stripped off is then subjected tochromatographic purification on silica gel usingdichloromethane/MeOH/triethylamine (100/10/1). The product-containingfractions are combined and concentrated. 0.81 g of a colorless foamremains.

R_(f) =0.29 (dichloromethane/methanol/triethylamine 100:10:1).

MS(ES⁻): 1112.0 (2M-H)⁻, 555.3 (M-H)⁻.

Example 24N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-thyminyl)acetyl)glycinemethyl ester

(Mmt-Aeg(T)-OMe)

1.00 g (2.48 mmol) ofN-((4-Methoxyphenyl)diphenylmethyl)aminoethylglycine methyl ester isdissolved in 5 ml of DMF. 0.404 g (2.48 mmol) of3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine and 0.632 ml (4.96 mmol)of 4-ethylmorpholine are added to this solution. A solution of 0.456 g(2.48 mmol) of N-1-carboxymethylthymine in 5 ml of DMF is then addeddropwise, followed by 0.46 ml (3.0 mmol) ofN,N'-diisopropylcarbodiimide. The reaction mixture is stirred for 20hours at room temperature. The solvent is then removed in vacuo and theresidue dissolved in ethyl acetate. This solution is washed twice withwater and saturated potassium chloride solution. The organic phase isdried over sodium sulfate, filtered and then evaporated to dryness. Theresulting crude product is purified chromatographically on silica gelwhich has been equilibrated with a mixture ofdichloromethane/methanol/triethylamine (100/1/1), with a gradient of1-5% of methanol in dichloromethane. The product-containing fractionsare combined and concentrated in vacuo. 1.28 g of product are obtainedas a colorless foam.

MS(FAB): 571.2 (M+H)⁺.

R_(f) =0.28 (CH₂ Cl₂ :MeOH/95:5).

Example 25N-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-thyminyl)acetyl)glycine

(Mmt-Aeg(T))-OH

The product of the above reaction is dissolved in a mixture of 10 ml ofdioxane and 2 ml of water. The solution is cooled to 0° C., and 1Nsodium hydroxide solution is added dropwise until a pH of 11 has beenreached. After a reaction time of 2 hours, the reaction is complete, andthe pH of the solution is brought to 5 by carefully adding 2N KHSO₄solution. The solution is extracted three times using ethyl acetate, andthe combined organic phases are dried over sodium sulfate andconcentrated in vacuo. The resulting crude product is purifiedchromatographically on silica gel with a gradient of 5-10% of methanoland 1% of triethylamine in dichloromethane. The product-containingfractions are combined and concentrated in vacuo. Excess triethylaminewhich is still present is removed by coevaporation with pyridine andthen toluene. 1.065 g of product are obtained as a colorless foam.

MS(ES⁻): 1112.0 (1M-H)⁻, 555.3 (M-H)⁻.

R_(F) =0.28 (CH₂ Cl₂ :MeOH/8:2).

Example 26 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-benzoyl)cytosyl)acetyl)glycine methyl ester

(Mmt-Aeg(C^(Bz))-OMe)

1.10 g N-((4-methoxyphenyl)diphenylmethyl)aminoethylglycine methyl ester(2.72 mmol) are dissolved in 5 ml of DMF and 0.444 g (2.72 mmol) of3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine and 0.94 ml of4-ethylmorpholine (5.45 mmol) are added. A suspension of 0.744 g (2.72mmol) of N⁴ -benzoyl-N¹ -carboxymethylcytosine in 20 ml of DMF is added,followed by 0.51 ml (3.27 mmol) of N,N'-diisopropylcarbodiimide. Thereaction mixture is stirred for 20 hours at room temperature. Thesolvent is then stripped off in vacuo and the residue dissolved in ethylacetate. This solution is washed twice using water and once usingsaturated sodium chloride solution. The organic phase is dried oversodium sulfate, filtered and concentrated in vacuo. The resulting crudeproduct is purified chromatographically on silica gel which has beenequilibrated with a mixture of dichloromethane/methanol/triethylamine(100/1/1) with a gradient of 1-2% of methanol in dichloromethane. Theproduct-containing fractions are combined and concentrated in vacuo.0.96 g of product is obtained as a yellowish white foam.

MS(ES⁻) 658.6 (M-H)⁻.

R_(f) =0.37 (CH₂ Cl₂ :MeOH/95:5).

Example 27 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-benzoyl)cytosyl)acetyl)glycine

(Mmt-Aeg(C^(Bz)))-OH

2.26 g (3.43 mmol) ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-benzoyl)cytosyl)acetyl)glycine methyl ester are dissolved in 50 ml ofdioxane. The solution is cooled to 0° C., and 34.4 ml of 1N sodiumhydroxide solution are added dropwise. After 10 minutes, the pH isbrought to 5 by dropwise addition of 1N KHSO₄, and salts which haveprecipitated are filtered off and washed with a small amount of dioxane.The combined filtrates are evaporated in vacuo, and the residue iscoevaporated twice with methanol and dichloromethane. The resultingcrude product is purified chromatographically on silica gel with agradient of 5-10% of methanol and 1% of triethylamine indichloromethane. The product-containing fractions are combined andconcentrated in vacuo. Any excess triethylamine which is still presentis removed by coevaporation with pyridine and then toluene. 1.306 g ofproduct are obtained as a virtually white foam.

MS(ES⁻) 644.3 (M-H)⁻.

R_(f) =0.68 (CH₂ Cl₂ :MeOH/8:2).

Example 28 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-(4-tert-butylbenzoyl)cytosyl)acetyl)glycine methyl ester

(Mmt-Aeg(C^(tBuBz))-OMe)

1.00 g (2.48 mmol) ofN-((4-methoxyphenyl)diphenylmethyl)aminoethylglycine methyl ester isdissolved in 5 ml of DMF. 0.403 g (2.48 mmol) of3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine and 0.63 ml (4.96 mmol)of 4-ethylmorpholine are added in succession to this solution. Asuspension of 0.740 g (2.48 mmol) of N⁴ -(4-tert-butylbenzoyl)-N-carboxymethylcytosine in 5 ml of DMF is then added, followed by 0.47 mlof N,N'-diisopropylcarbodiimide. The mixture is stirred for 20 hours atroom temperature and then evaporated to dryness in vacuo. The residue isdissolved in ethyl acetate, washed with water and saturated sodiumchloride solution, dried over sodium sulfate, filtered and concentratedin vacuo. The crude product is purified using a silica gel column whichhas been equilibrated with a mixture of dichloromethane/ethylacetate/triethylamine (50/50/1), using dichloromethane/ethyl acetate(1/1) as the eluent. The combined, product-containing fractions areconcentrated in vacuo. The product is obtained as a yellowish white foamin a yield of 1.635 g.

MS(FAB/MeOH,NBA,LiCl): 722.5 (M+Li)⁺.

R_(f) =0.31 (CH₂ Cl₂ :ethyl acetate/1:1).

Example 29 N-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-(4-tert-butylbenzoyl)cytosyl)acetyl)glycine

(Mmt-Aeg(C^(tBuBz))-OH)

1.63 g (2.28 mmol) ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-(4-tert-butylbenzoyl)cytosyl)acetyl)glycine methyl ester is dissolvedin a mixture of 10 ml of dioxane and 1 ml of water and the mixture istreated dropwise with 4.56 ml of 1N NaOH at 0° C., with stirring. After2 hours, the pH is brought to 5 by dropwise addition of 1N KHSO₄, andprecipitated salts are filtered off and washed with a small amount ofdioxane. The combined filtrates are evaporated in vacuo and the residueis coevaporated twice with methanol and dichloromethane. The resultingcrude product is purified chromatographically on silica gel with agradient of 2-10% of methanol and 1% of triethylamine indichloromethane. The product-containing fractions are combined andconcentrated in vacuo. Any excess triethylamine which is still presentis removed by coevaporation with pyridine and subsequently toluene.0.831 g of product is obtained as a virtually white foam.

MS(ES⁻) 700.7 (M-H)⁻.

R_(f) =0.28 (CH₂ Cl₂ :MeOH/9:1), 0.63 (CH₂ Cl₂ :MeOH/7:3).

Example 30N-((4-Methoxyphenyl)diphenylmethyloxy)ethyl-N-((1-thyminyl)acetyl)glycine

(Mmt-Oeg(T)-OH)

0.5 g (1.28 mmol) of N-((4-methoxyphenyl)diphenylmethyloxy)ethylglycineis suspended in 10 ml of DMF, and 0.47 ml (1.92 mmol) of BSA is addeddropwise. 0.7 ml (5.1 mmol) of triethylamine and 0.26 g (1.28 mmol) ofchlorocarboxymethylthymine are then added in succession. The reactionmixture is stirred for 4 hours at room temperature, a further 65 mg(0.32 mmol) of chlorocarboxymethylthymine are then added, and themixture is stirred for a further 16 hours. The solvent is then strippedoff in vacuo and the crude product is purified on a silica gel columnwith a gradient of 5-15% of methanol and 1% of triethylamine indichloromethane. The product-containing fractions are combined andconcentrated in vacuo. The brownish oil obtained is dissolved in a smallamount of dichloromethane and the product is precipitated by addingdiethyl ether. The product is obtained as a virtually white powder.

Yield: 0.219 g.

MS(ES⁻) 556.3 (M-H)⁻.

R_(f) =0.54 (CH₂ Cl₂ :MeOH/8:2).

Example 31 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-(4-methoxybenzoyl)cytosyl)acetyl)glycine methyl ester

(Mmt-Aeg(C^(MeOBz))-OMe)

N-((4-Methoxyphenyl)diphenylmethyl)aminoethylglycine methyl ester (4.00g; 9.9 mmol) is dissolved in DMF (50 ml). To this solution there areadded, in succession, 4-ethylmorpholine (3.44 ml),3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine (1.62 g) N⁴-(4-methoxybenzoyl)-N¹ -carboxymethylcytosin (3.00 g; 9.9 mmol) andN,N'-diisopropylcarbodiimide (1.87 ml). The reaction mixture is stirredfor 48 hours at room temperature. The solvent is then evaporated invacuo and the residue taken up in ethyl acetate. This solution isextracted twice using water and once using saturated potassium chloridesolution. The organic phase is dried over sodium sulfate, filtered andconcentrated in vacuo. Purification is effected by means of columnchromatography on silica gel which has previously been equilibrated withdichloromethane containing 1% of triethyl amine. Elution is effectedwith a gradient of 0-2% of methanol in dichloromethane containing 1% oftriethylamine. The product-containing fractions are combined andconcentrated in vacuo to a small volume. The dicyclohexylurea whichprecipitates during this process is filtered off, and the filtrate isevaporated in vacuo. The compound desired is obtained as a pale yellowfoam.

Yield: 6.72 g.

MS (FAB/MeOH,NBA,LiCl): 696.3 (M+Li)⁺.

R_(f) =0.59 (dichloromethane:methanol/9:1).

Example 32 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(N⁴-(4-methoxybenzoyl)cytosyl)acetyl)glycine

(Mmt-Aeg(C^(MeOBz))-OH)

N-((4-Methoxyphenyl)diphenylmethyl)aminoethyl-N-((1-(N⁴-(4-methoxybenzoyl))cytosyl)acetyl)glycine methyl ester (6.60 g; 9.6mmol) is dissolved in a mixture of dioxane (50 ml) and water (50 ml),and the mixture is treated dropwise at 0° C. with 1M aqueous sodiumhydroxide solution (15 ml) in two portions. After a reaction time of 2hours, the solution is rendered neutral by adding ion exchanger (DowexAG 50WX4, pyridinium form). The ion exchanger is filtered off and washedwith aqueous dioxane and methanol. The combined filtrates are evaporatedto dryness in vacuo and purified by column chromatography on silica gelby elution using 10% of methanol in dichloromethane (with 1% oftriethylamine). The product is obtained as a yellowish white foam. Acolorless powder is obtained after recrystallization from ethyl acetate.

Yield: 4.00 g.

MS(ES⁻): 674.1 (M-H)⁻.

R_(f) =0.39 (dichloromethane:methanol/9:1).

Example 33 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-(9-(N²-(isobutanoyl)guanosyl)acetyl)glycine methyl ester

(Mmt-Aeg(G^(iBu))-OMe)

N-((4-Methoxyphenyl)diphenylmethyl)aminoethylglycine methyl ester (1.45g; 3.59 mmol) is dissolved in DMF (7 ml). To this solution there areadded, in succession, 4-ethylmorpholine (1.24 ml; 7.17 mmol),3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine (0.585 g; 3.59 mmol), N²-(isobutanoyl)-N⁹ -carboxymethylguanine (1.00 g; 3.59 mmol) andN,N'-diisopropylcarbodiimide (0.67 ml: 4.31 mmol). The reaction mixtureis stirred for 48 hours at 4° C. The solvent is then evaporated in vacuoand the residue taken up in ethyl acetate. This solution is extractedtwice using water and once using saturated potassium chloride solutionand the organic phase is dried over sodium sulfate, filtered andconcentrated in vacuo. The yellow foam which remains is dissolved in asmall amount of ethyl acetate and cooled with ice to precipitatedicyclohexylurea which is still present. After filtration, the filtrateis concentrated in vacuo. Purification is effected by means of columnchromatography on silica gel which has previously been equilibrated withdichloromethane:ethyl acetate 1/1 containing 1% of triethylamine.Elution is effected using dichloromethane:ethyl acetate 1/1. Theproduct-containing fractions are combined and concentrated in vacuo. Thecompound desired is obtained as a pale yellow foam.

Yield: 1.181 g.

MS (FAB/MeOH,NBA,LiCl): 678.3 (M+2Li-H)⁺ ; 672.3 (M+Li)⁺.

R_(f) =0.13 (dichloromethane:methanol/95:5).

Example 34 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-(9-(N²-(isobutanoyl)guanosyl)acetyl)glycine

(Mmt-Aeg(G^(iBu))-OH)

N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-(9-(N²-(isobutanoyl)guanosyl)acetyl)glycine methyl ester (1.15 g; 1.72 mmol)is dissolved in dioxane (10 ml), and the solution is treated dropwisewith 1M aqueous sodium hydroxide solution (10.32 ml) in 5 portions at 0°C. over a period of 2.5 hours. After a further reaction time of 2 hoursat room temperature, the pH of the solution is brought to 5 by dropwiseaddition of 2M aqueous potassium hydrogen sulfate solution. The saltswhich have precipitated are filtered off and washed with a small amountof dioxane. The combined filtrates are evaporated to dryness in vacuo,and the residue is coevaporated in each case twice with ethanol and withdichloromethane:methanol 1/1. Purification is effected by columnchromatography on silica gel by elution using a gradient of 10-20% ofmethanol in dichloromethane (with 1% of triethylamine). The product isobtained as a white foam.

Yield: 1.229 g.

MS(ES⁻): 650.3(M-H)⁻.

R_(f) =0.25 (dichloromethane:methanol/8:2).

Example 35 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-(9-(N⁶-(4-methoxybenzoyl)adenosyl)acetyl)glycine methyl ester

(Mmt-Aeg(A^(MeOBz))-OMe)

N-((4-Methoxyphenyl)diphenylmethyl)aminoethylglycine methyl ester (1.24g; 3.06 mmol) is dissolved in DMF (7 ml). To this solution there areadded, in succession, 4-ethylmorpholine (1.06 ml; 6.12 mmol),3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine (0.498 g; 3.06 mmol), N⁶-(4-methoxybenzoyl)-N⁹ -carboxymethyladenine (1.00 g; 3.06 mmol) andN,N'-diisopropylcarbodiimide (0.58 ml; 3.67 mmol).

The reaction mixture is stirred for 48 hours at 4° C. The solvent isthen evaporated in vacuo and the residue taken up in ethyl acetate. Thissolution is extracted twice using water and once using saturatedpotassium chloride solution. The organic phase is dried over sodiumsulfate, filtered and concentrated in vacuo. The yellow foam whichremains is dissolved in a small amount of ethyl acetate and cooled withice to precipitate dicyclohexylurea which is still present. Thedicyclohexylurea is filtered off and the filtrate concentrated in vacuo.Purification is effected by means of column chromatography on silica gelwhich has previously been equilibrated with dichloromethane:ethylacetate 1/1 containing 1% of triethylamine. Elution is effected usingdichloromethane:ethyl acetate 1/1. The product-containing fractions arecombined and concentrated in vacuo. The desired compound is obtained asa pale yellow foam.

Yield: 1.752 g.

MS (FAB/MeOH,NBA,LiCl): 720.3 (M+Li)⁺.

R_(f) =0.21 (dichloromethane:methanol/95:5).

Example 36 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-(9-(N⁶-(4-methoxybenzoyl)adenosyl)acetyl)glycine

(Mmt-Aeg(A^(MeOBz))-OH)

N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-(9-(N⁶-(4-methoxybenzoyl)adenosyl)acetyl)glycine methyl ester (1.70 g; 2.38mmol) is dissolved in dioxane (10 ml) and the solution is treateddropwise with 1M aqueous sodium hydroxide solution (10.32 ml) in 5portions at 0° C. over a period of 2.5 hours. After a further reactiontime of 2 hours at room temperature, the pH of the solution is broughtto 5 by dropwise addition of 2M aqueous potassium hydrogen sulfatesolution. The salts which have precipitated are filtered off and washedwith a small amount of dioxane. The combined filtrates are evaporated todryness in vacuo and the residue is coevaporated in each case twice withethanol and dichloromethane:methanol 1/1. Purification is effected bycolumn chromatography on silica gel by elution using a gradient of10-20% of methanol in dichloromethane (with 1% of triethylamine). theproduct is obtained as a white foam.

Yield: 1.619 g.

MS(ES⁻): 698.3(M-H)⁻.

R_(f) =0.10 (dichloromethane:methanol/8:2).

Example 37 Thyminylacetic acid

Preparation process as described by L. Kosynkina, W. Wang and T. ChyauLiang, Tetrahedron Lett. 1994, 5173-5176

37.8 g of thymine are dissolved in a solution of 64.5 g of potassiumhydroxide in 200 ml of water. A solution of 62.5 g of bromoacetic acidin 100 ml of water is then added dropwise at 40° C. in the course of 70minutes, with vigorous stirring. Stirring is continued for 1 hour at 40°C., the solution is allowed to cool to 20° C., and the pH is brought to5.5 by adding concentrated hydrochloric acid. The mixture is allowed tostand for 1.5 hours at 0° C., the unreacted thymine which hasprecipitated is filtered off with suction, and the pH of the filtrate isthen brought to 2.0 using concentrated hydrochloric acid, during whichprocess the product precipitates. The product is allowed to stand for afurther hour at 0° C., and the precipitate is then filtered off withsuction and dried in a desiccator.

Yield: 52.19 g.

R_(f) =0.23 (ethyl acetate/methanol/glacial acetic acid 75:20:5).

MS(DCI): 185(M+H)⁺.

Example 38 Methyl thyminylacetate

31.5 g of thymine are suspended in 800 ml of dry DMF, and 69.4 g offinely pulverulent potassium carbonate are added. The mixture is shakenfor 3 hours at room temperature, and 23.1 ml of methyl bromoacetate arethen added. The mixture is shaken overnight under argon, the precipitateis then filtered off and washed with DMF, and the filtrate isconcentrated in vacuo on a rotary evaporator. The residue is trituratedthoroughly in a mortar in a mixture of 240 ml of water and 19 ml of 2NHCl, and the precipitate is then filtered off with suction and washedwith water. The resulting crude product, which is still moist withwater, is recrystallized from 450 ml of methanol.

Yield: 25.5 g.

R_(f) =0.78 (ethyl acetate/methanol/glacial acetic acid 80:10:5).

MS(DCI): 199.1 (M+H)⁺.

Example 39N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-thyminyl)acetyl)glycinemethyl ester

Mmt-Aeg(T)-OMe

19.84 g of Mmt-Aeg-OMe are dissolved in 450 ml of THF, and then 6.82 mlof triethylamine are first added with cooling and vigorous stirring,followed by 3.92 ml of chloroacetyl chloride, slowly added dropwise,dissolved in 60 ml of THF. After approximately 2/3 of the chloroacetylchloride solution have been added dropwise, a further 3.41 ml oftriethylamine are added. After the addition of chloroacetyl chloride hasended, the mixture is allowed to afterreact for one hour, precipitatedtriethylamine hydrochloride is filtered off with suction, and thesolution is treated with 400 ml of dry DMF. The THF is subsequentlystripped off in vacuo on a rotary evaporator, and this solution ofMmt-Aeg(chloroacetyl)-OMe in DMF is added to a mixture of 12.36 g ofthymine and 27.16 g of finely pulverulent potassium carbonate in 200 mlof dry DMF, which has already pre-reacted overnight. This mixture isstirred for a further 16 hours at room temperature, and undissolvedmatter is then filtered off with suction. The filtrate is concentratedin vacuo on a rotary evaporator, and the residue is partitioned betweenwater and ethyl acetate. The organic phase is dried over sodium sulfate,the dessicant is filtered off in vacuo and the filtrate is thenconcentrated on a rotary evaporator to a volume of approximately 80 ml.This solution is then stirred into a mixture of 600 ml of petroleumether and 200 ml of diethyl ether, during which process the productprecipitates.

Yield: 25 g.

R_(f) =0.34 (dichloromethane/methanol/triethylamine 100:5:1).

Example 40N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-thyminyl)acetyl)glycine

Mmt-Aeg(T)-OH

5.7 g of Mmt-Aeg(T)-OMe are suspended in 25 ml of water, and thesuspension is treated in succession with 13.9 ml of triethylamine and12.5 ml of dioxane. The resulting clear solution is allowed to stand for72 hours at room temperature, and the solvent is then distilled off invacuo on a rotary evaporator. This process is followed by threedistillations using small amounts of toluene. The residue is taken up ina mixture of in each case 20 ml of dioxane and ethyl acetate, a smallamount of undissolved matter is filtered off, and the filtrate isstirred into 500 ml of ether containing 1% of triethylamine. The productwhich has precipitated is filtered off with suction and dried in adesiccator.

Yield: 4.1 g.

R_(f) =0.28 (dichloromethane/methanol/triethylamine 100:10:1).

Example 41N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((1-thyminyl)acetyl)glycine

Mmt-Aeg(T)-OH

5.7 g Mmt-Aeg(T)-OMe are dissolved in a mixture of 80 ml of water, 20 mlof methanol and 13.9 ml of triethylamine. The resulting clear solutionis stirred for 24 hours at room temperature, and the solvent is thendistilled off in vacuo on a rotary evaporator. This is followed by threedistillations using small amounts of toluene. The residue is taken up ina mixture of in each case 20 ml of dioxane and ethyl acetate, a smallamount of undissolved matter is filtered off, and the filtrate isstirred into 500 ml of ether containing 1% of triethylamine. The productwhich has precipitated is filtered off with suction and dried in adesiccator.

Yield: 4.8 g.

R_(f) =0.28 (dichloromethane/methanol/triethylamine 100:10:1).

Example 42 N-((2-Hydroxy)ethyl-N-((1-thyminyl)acetyl)glycine

H-Oeg(T)-OH

2.76 g of thyminylacetic acid are dissolved in 15 ml of dry DMF, and4.92 g of TOTU and 2.08 ml of triethylamine are added. Stirring of themixture is continued for 30 minutes at room temperature, and the mixtureis then slowly added dropwise to a solution composed of 3.57 g of(2-hydroxyethyl)glycine, 10 ml of water, 10 ml of DMF and 4.16 ml oftriethylamine. Stirring is continued for 1 hour at room temperature, and20 ml of acetonitrile are then added, during which process the excesshydroxyethylglycine precipitates. The precipitate is filtered off withsuction, and the filtrate is concentrated in vacuo on a rotaryevaporator. The residue is taken up in water, the pH is brought to 1.5using 1N hydrochloric acid, and the mixture is extracted using ethylacetate. The pH of the aqueous phase is brought to 5 using saturatedsodium hydrogen carbonate solution and it is concentrated on a rotaryevaporator. The residue is treated with 250 ml of ethanol, and thesodium chloride which has precipitated in this process is filtered offwith suction. The filtrate is concentrated, and the crude product ispurified chromatographically on silica gel usingdichloromethane/methanol/ethyl acetate 10:2:1 with an addition of 1% oftriethylamine. The product-containing fractions are combined andconcentrated in vacuo on a rotary evaporator.

Yield: 2.39 g.

R_(f) =0.17 (dichloromethane/methanol/ethyl acetate 10:2:1+1% oftriethylamine).

MS(DCl): 286(M+H)⁺.

Example 43N-(2-(di-(4-Methoxyphenyl)phenyl)methyloxy)ethyl-N-((1-thyminyl)acetyl)glycine

Dmt-Oeg(T)-OH

1.76 g of H-Oeg(T)-OH are dissolved in 30 ml of DMF, 3.41 ml oftriethylamine are added, and a solution of 4.16 g of Dmt-Cl in 30 ml ofdichloromethane is added dropwise at 0° C. in the course of 20 minutes.Stirring is continued for 3 hours at room temperature, the triethylaminehydrochloride which has precipitated is then filtered off, and thefiltrate is concentrated in vacuo on a rotary evaporator. The crudeproduct is purified on silica gel using dichloromethane/methanol/ethylacetate 10:2:1 with an addition of 1% of triethylamine. Theproduct-containing fractions are combined and concentrated in vacuo on arotary evaporator.

Yield: 3.2 g.

R_(f) =0.29 (dichloromethane/methanol/ethyl acetate 10:2:1+1%triethylamine).

MS(ES⁺ +LiCl): 594.3(M+Li)+, 600.4(M+2Li-H)⁺.

Example 44 Preparation ofN-(2-(di-(4-Methoxyphenyl)phenyl)methylthio)ethyl-N-((1-thyminyl)acetyl)glycine

Dmt-Teg(T)-OH

Example 44a

Dmt-S-CH₂ --CH₂ --NH₂

10.17 g of Dmt-Cl are dissolved in 100 ml of acetic acid, and 4.43 g of2-mercaptoethylamine hydrochloride, dissolved in 70 ml of water, areadded. Stirring is continued for 2 hours at room temperature, and themixture is then concentrated on a rotary evaporator to a volume ofapproximately 50 ml. 200 ml of water are then added, and the pH of thesolution is brought to 10 using 2N NaOH, during which process thesubstance precipitates. The mixture is extracted 3 times using 100 ml ofethyl acetate in each case, and the combined organic phases are washedwith saturated NaCl solution and dried over sodium sulfate. Thedesiccant is filtered off, and the filtrate is concentrated in vacuo ona rotary evaporator. The resulting crude product is employed in thesubsequent reaction without further purification.

Yield: 14.75 g.

Example 44b

Dmt-S-CH₂ --CH₂ --NH--CH₂ --CO₂ C₂ H₅

12.56 g of Dmt-S-CH₂ --CH₂ --NH₂ (crude product of 44a) are dissolved in100 ml of dry DMF, and the solution is treated in succession withstirring at room temperature with 4.6 ml of triethylamine and 3.66 ml ofethyl bromoacetate. Stirring is continued for 3 hours, a small amount ofprecipitate is filtered off, and the filtrate is concentrated in vacuoon a rotary evaporator. The residue is taken up in 150 ml of ethylacetate and washed 4 times using 40 ml of water in each case, theorganic phase is dried over sodium sulfate, the desiccant is filteredoff, and the filtrate is then concentrated in vacuo on a rotaryevaporator. The resulting crude product is employed in the subsequentreaction without further purification.

Yield: 12.92 g.

Example 44c

Dmt-Teg(T)-OEt

5.95 g of Dmt-S-CH₂ --CH₂ --NH--CH₂ --CO₂ C₂ H₅ are dissolved in 50 mlof dry DMF and treated in succession with 2.36 g of thyminylacetic acid,4.45 ml of triethylamine and 4.2 g of TOTU. The mixture is stirred for 2hours at room temperature and allowed to stand overnight. The reactionmixture is then concentrated in vacuo on a rotary evaporator, theresidue is taken up in 150 ml of ethyl acetate, the mixture is washed 5times using in each case 10 ml of saturated sodium hydrogen carbonatesolution and twice using 10 ml of water, and the organic phase is driedover sodium sulfate. The desiccant is filtered off, and the filtrate isconcentrated in vacuo on a rotary evaporator. An oily crude product isobtained which is employed in the subsequent reaction without furtherpurification.

Yield: 8.06 g.

Example 44d

Dmt-Teg(T)-OH

8.06 g of Dmt-Teg(T)-OEt are dissolved in a mixture of 80 ml of dioxaneand 40 ml of water and hydrolyzed by adding a total of 13 ml of 2Nsodium hydroxide solution in portions. The solution is then concentratedin vacuo on a rotary evaporator to approximately 50 ml and extractedfour times using in each case 50 ml of ethyl acetate. The pH of theaqueous phase is then brought to 5 using 2N hydrochloric acid andextracted four times using 50 ml of ethyl acetate. The combined organicphases are washed twice using in each case 30 ml of water and dried oversodium sulfate. The desiccant is filtered off, and the filtrate isconcentrated in vacuo on a rotary evaporator to a volume ofapproximately 30 ml. A small amount of triethylamine is added, and thesolution is stirred into 300 ml of diisopropyl ether, during whichprocess the product precipitates. To purify the product further, it ischromatographed on silica gel using a step gradient of 0-5% of methanolin dichloromethane (1% of triethylamine in all eluents). Theproduct-containing fractions are combined and concentrated in vacuo on arotary evaporator.

Yield: 2.38 g.

R_(f) =0.38 (dichloromethane/methanol/triethylamine 100:10:1).

MS(FAB/NBA+LiCl): 610.2(M+Li)⁺, 616.2(M+2Li-H)⁺.

Example 452-N-Acetyl-4-O-diphenylcarbamoyl-9-methoxycarbonylmethylguanine

15.52 g of 2-N-acetyl-4-O-diphenylcarbamoylguanine (prepared asdescribed by R. Zou and M. J. Robins, Can J. Chem. 65, 1436-1437, 1987)are suspended in 200 ml of dry DMF, 13.6 ml of diisopropylethylamine areadded, and the mixture is then heated briefly until a clear solution hasformed. 4.04 ml of methyl bromoacetate are then added and the solutionis stirred overnight. The solvent is subsequently removed in vacuo on arotary evaporator and the residue dissolved in 200 ml of methanol. Thissolution is stirred into 600 ml of water with vigorous stirring, duringwhich process the product precipitates. The resulting crude product isfiltered off with suction, washed with water and redissolved inmethanol, the solution is concentrated to dryness, and the product istriturated with ethyl acetate. The resulting product is filtered offwith suction, washed with ethyl acetate and ether and then dried invacuo. More product is obtained from the mother liquor afterconcentration and retrituration with ethyl acetate.

Yield: 13.2 g.

MS(ES⁺): 461.3(M+H)⁺.

R_(f) =0.79 (2-butanone/pyridine/water/acetic acid 70:15:15:2).

Example 46 2-N-Acetyl-4-O-diphenylcarbamoyl-9-carboxymethylguanine

7.54 g of2-N-acetyl-4-O-diphenylcarbamoyl-9-methoxycarbonylmethylguanine aredissolved in a mixture of 20 ml of methanol, 80 ml of dioxane and 40 mlof water and hydrolyzed with a total of 18 ml of 1N NaOH at a pH of 13.The pH of the solution is then brought to 6 by adding 1N hydrochloricacid and the mixture is evaporated in vacuo on a rotary evaporator to avolume of approximately 50 ml. After 400 ml of water have been added,the pH is brought to 3 using 1N hydrochloric acid, during which processthe product precipitates. The product is filtered off with suction,washed with water and dried in a desiccator.

Yield: 5.92 g.

MS(ES⁺): 447.2(M+H)⁺.

R_(f) =0.48 (2-butanone/pyridine/water/acetic acid 0:15:15:2).

Example 47 N-((4-Methoxyphenyl)diphenylmethylamino)ethyl-N-((9-(N²-acetyl-O⁴ -diphenylcarbamoyl)guanosyl)acetyl)glycine methyl ester

Mmt-Aeg(G^(2-Ac),4-Dpc)-OMe

3.57 g of 2-N-acetyl-4-O-diphenylcarbamoyl-9-carboxymethylguanine aredissolved in 120 ml of dry DMF, and 3.23 g of Mmt-Aeg-OMe, 2.62 g ofTOTU and, in portions, 4.08 ml of diisopropylethylamine are added insuccession. The mixture is stirred for 4 hours at room temperature andleft to stand overnight. The solvent is then distilled off in vacuo on arotary evaporator and the residue is partitioned between 160 ml ethylacetate and 20 ml of water. The organic phase is extracted three timesusing in each case 20 ml of water and dried over sodium sulfate, thedesiccant is filtered off, and the filtrate is concentrated. The residuewhich remains is triturated repeatedly with diethyl ether, and theproduct which has then separated out as a precipitate is filtered offwith suction and dried in a desiccator.

Yield: 5.53 g.

R_(f) =0.63 (dichloromethane/methanol/triethylamine 100:5:1).

Example 48 N-((4-Methodyphenyl)diphenylmethylamino)ethyl-N-((9-(N²-acetyl-O-diphenylcarbamoyl)guanosyl)acetyl)glycine

Mmt-Aeg(G^(2-Ac),4-Dpc)-OH

4.99 g Mmt-Aeg(G^(2-Ac),4-Dpc)-OMe are dissolved in a mixture of 50 mlof dioxane and 10 ml of water and hydrolyzed by adding a total of 15 mlof 0.5N NaOH in portions. The pH of the solution is then brought to 9using 0.5N HCl and the solvent is stripped off in vacuo on a rotaryevaporator. This is followed by one more distillation with a smallamount of toluene, and the residue is then dissolved in 30 ml ofmethanol. This solution is stirred into 300 ml of diethyl ether (whichhas been treated with 1% of triethylamine), during which process theproduct precipitates. The precipitate, which is still somewhat tacky, istriturated with diethyl ether, filtered off with suction and dried in adesiccator. It is purified further by means of chromatography on silicagel using a step gradient (4-16%) of methanol/ethyl acetate (1:1) indichloromethane, with 1% of triethylamine in all solvents. Theproduct-containing fractions are combined and concentrated in vacuo on arotary evaporator.

Yield: 2.68 g.

R_(f) =0.18 (dichloromethane/methanol/triethylamine 100:7.5:1).

MS(ES⁺ +LiCl): 819.4(M+H)⁺, 825.3(M+Li)⁺.

Example 49 1-Acetyl-2,4-dihydroxy-5-methylpyrimidine

1-Acetylthymine

75 g of thymine are suspended in 375 ml of acetic anhydride and refluxedfor 45 minutes. The mixture is then cooled to 0° C., during whichprocess the product precipitates. The precipitate is filtered off withsuction and stirred with 375 ml of ethyl acetate. The precipitate isfiltered off with suction, washed with a small amount of diethyl etherand dried.

Yield: 89.1 g.

Melting point: 187-189° C.

R_(f) =0.67 (dichloromethane/methanol 95:5).

Example 50 3-Benzyloxymethyl-2,4-dihydroxy-5-methylpyrimidine

3-Benzyloxymethylthymine

52.0 g of 1-acetyl-2,4-dihydroxy-5-methylpyrimidine are suspended in 300ml of DMF, 47.5 ml of triethylamine are added, and the mixture is cooledto 0° C. 100 ml of benzyl chloromethyl ether in 50 ml of DMF are slowlyadded dropwise to the thoroughly stirred mixture at this temperature.Stirring is continued for 1 hour and the mixture is left to standovernight at room temperature. The reaction mixture is then concentratedin vacuo on a rotary evaporator, taken up in 450 ml of methanol and 300ml of a 40% aqueous solution of methylamine and refluxed for 1.5 hours.The solution is then concentrated and stirred with 10.8 g of sodiumhydrogen carbonate in 600 ml of water. The product which hasprecipitated is filtered off with suction, washed with water and ethanoland recrystallized from ethanol.

Yield: 46.66 g.

Melting point: 119-120° C.

R_(f) =0.38 (dichloromethane/methanol 95:5).

Example 51 Ethyl3-benzyloxymethyl-2,4-dihydroxy-5-methylpyrimidineacetate

3-Benzyloxymethyl-1-ethoxycarbonylmethylthymine

4.97 g of sodium hydride are introduced into 150 ml of THF, 45.04 g of3-benzyloxymethyl-2,4-dihydroxy-5-methylpyrimidine, dissolved in 550 mlof THF, are then added dropwise at 0° C. under an N₂ atmosphere, and21.42 ml of ethyl bromoacetate in 700 ml of THF are subsequently addedat this temperature. Thereinafter, stirring of the mixture is continuedfor 5 hours at room temperature, and water is subsequently addedcarefully. The organic phase is separated off, and the aqueous phase isreextracted four times using dichloromethane. The combined organicphases are dried over magnesium sulfate and concentrated, and theresidue is stirred with heptane.

Yield: 54.2 g.

R_(f) =0.15 (dichloromethane/methanol 95:5).

MS(FAB, NBA): 319.1(M+H)⁺.

Example 52 Ethyl 2,4-dihydroxy-5-methylpyrimidineacetate

Ethyl thyminylacetate

25 g of ethyl 3-benzyloxymethyl-2,4-dihydroxy-5-methylpyrimidineacetateare dissolved in 1000 ml of dichloromethane, the mixture is cooled to-70° C., and 375 ml of a 1M solution of boron trichloride indichloromethane is added dropwise at this temperature with vigorousstirring. The mixture is allowed to afterreact for 3 hours at thistemperature, whereupon 320 ml of methanol are added dropwise at -70° to-60° C. After 1 hour, 197.05 ml of triethylamine are added and themixture is allowed to come to room temperature. A small amount of wateris then also added and the mixture is concentrated on a rotaryevaporator. The residue is taken up in ethyl acetate and washed withwater. The organic phase is dried over magnesium sulfate and thenconcentrated to dryness.

Yield: 12 g.

R_(f) =0.45 (dichloromethane/methanol 9:1).

MS(DCI): 199(M+H)⁺.

Example 53 2,4-Dihydroxy-5-methylpyrimidineacetic acid

Thyminylacetic acid

10 g of ethyl 2,4-dihydroxy-5-methylpyrimidineacetate are dissolved in130 ml of dioxane/water (1:1), 60 ml of 1N LiOH are added and themixture is then stirred overnight at room temperature. The mixture isthen concentrated to a small volume, the aqueous phase is washed withether, and the pH is then brought to 2.5. During this process, theproduct precipitates. It is filtered off with suction, 4.9 g beingobtained. A further 1.5 g of product can be obtained from the motherliquor by extraction with pentanol and precipitation with heptane/ether.

Yield: 6.4 g.

R_(f) =0.30 (dichloromethane/methanol 3:2).

MS(DCI): 185(M+H)⁺.

Example 54 N-1-Chlorocarboxymethylthymine

Thyminylacetyl chloride

Example 54a

2 g of 2,4-dihydroxy-5-methylpyriminidineacetic acid and 15 ml ofthionyl chloride are stirred for 3 hours at 60° C., until the evolutionof gas has ceased. The excess thionyl chloride is then stripped off invacuo on a rotary evaporator, and this is followed by three moredistillations with small amounts of toluene. The resulting product isemployed directly in the subsequent reaction.

Yield: 2.4 g.

MS(DCI): 203(M+H)⁺.

Example 54b

N-1-Carboxymethylthymine (3.0 g; 16.3 mmol) is suspended in thionylchloride (90 ml). The suspension is heated at 70° C. for 1.5 hours andthen left to stand at room temperature for 16 hours. The excess thionylchloride is removed in vacuo and the residue coevaporated three timesusing toluene. The product obtained is triturated twice using n-hexaneand dried in vacuo. The compound is obtained as a pale orange powder.

Yield: 3.30 g.

MS(DCI; dichloromethane): 203 (M+H)⁺.

R_(f) =0.10 (dichloromethane/methanol 7:3).

Example 55 Preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-imidazolyl)acetyl)glycinemethyl ester

Mmt-Aeg(Im)-OMe

4.68 g of Mmt-Aeg-OMe are dissolved in 150 ml of THF, and 0.95 ml ofchloroacetyl chloride dissolved in 15 ml of THF simultaneously with 3.3ml of triethylamine dissolved in 10 ml of THF are slowly added dropwisewith cooling and vigorous stirring. The mixture is then allowed toafterreact for 45 minutes, precipitated triethylamine hydrochloride isfiltered off with suction, and the solution is treated with 150 ml ofdry DMF. The THF is subsequently stripped off in vacuo on a rotaryevaporator, and 1.62 g of imidazole and 6.55 g of finely pulverulentpotassium carbonate are added in succession to the solution ofMmt-Aeg(chloroacetyl)-OMe in DMF. After 16 hours, undissolved salt isfiltered off, and a further 1.62 g of imidazole and 6.55 g of finelypulverulent potassium carbonate are added to the filtrate. This mixtureis stirred for a further 16 hours at room temperature and undissolvedmatter is then filtered off with suction. The filtrate is concentratedin vacuo on a rotary evaporator and the residue partitioned betweenwater and ethyl acetate. The organic phase is dried over sodium sulfateand then evaporated after the desiccant has been filtered off. Theresidue is taken up in dichloromethane and precipitated by stirring intoa mixture of diethyl ether/petroleum ether 4:1. After the solvent hasbeen decanted off and the precipitate, still somewhat tacky, has beentriturated with ether, the precipitate solidifies.

Yield: 3.96 g.

R_(f) =0.39 (dichloromethane/methanol/triethylamine 100:5:1).

Example 56 Preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-imidazolyl)acetyl)glycine

Mmt-Aeg(Im)-OH

1.79 g ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-imidazolyl)acetyl)glycinemethyl ester are dissolved in a mixture of 50 ml of dioxane and 25 ml ofwater and hydrolyzed by adding 5.25 ml of 1N NaOH. The pH of thesolution is then brought to 8 using iN HCl and the mixture isconcentrated to dryness in vacuo on a rotary evaporator. The residue issubjected to two more distillations using small amounts of toluene andthen purified chromatographically on silica gel usingdichloromethane/methanol/ethyl acetate 100:10:5 with 1% oftriethylamine. The product-containing fractions are combined andconcentrated to dryness in vacuo on a rotary evaporator.

Yield: 1.70 g.

R_(f) =0.27 (dichloromethane/methanol/triethylamine 100:20:1).

MS(ES⁺): 499.3 (M+H)⁺.

Further syntheses involving a chloroacetyl intermediate step aredescribed hereinbelow:

Example 57 Preparation of the solution ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-(chloroacetyl)glycinemethyl ester in DMF

Mmt-Aeg(chloroacetyl)-OMe

34.85 g of Mmt-Aeg-OMe are dissolved in 900 ml of THF and then 6.9 ml ofchloroacetyl chloride, dissolved in 110 ml of THF, and 24 ml oftriethylamine, dissolved in 110 ml of THF, are simultaneously slowlyadded dropwise with cooling and vigorous stirring. The mixture is thenallowed to afterreact for 1 hour, precipitated triethylaminehydrochloride is filtered off with suction, and the solution is treatedwith 600 ml of dry DMF. The THF is subsequently stripped off in vacuo ona rotary evaporator. The resulting solution of Mmt-Aeg(chloroacetyl)-OMein DMF is divided.

Example 58 Preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(4-nitro)imidazolyl)acetyl)glycinemethyl ester

Mmt-Aeg(Im^(4-Nitro))-OMe

To one half of the above-prepared solution of Mmt-Aeg(chloroacetyl)-OMein DMF there are added 9.75 g of finely pulverulent 4-nitroimidazole and23.85 g of finely pulverulent potassium carbonate. This mixture isstirred for a further 16 hours at room temperature and undissolvedmatter is then filtered off with suction. The filtrate is concentratedin vacuo on a rotary evaporator and the residue is partitioned betweenwater and ethyl acetate. The organic phase is washed three times usingin each case 50 ml of water and dried over sodium sulfate, the desiccantis filtered off, and the filtrate is concentrated in vacuo on a rotaryevaporator to a volume of approximately 60 ml. This solution is thenstirred into a mixture of 400 ml of petroleum ether and 200 ml ofdiethyl ether, during which process the product precipitates. Theproduct is filtered off with suction and dried in a desiccator.

Yield: 19.8 g.

R_(f) =0.40 (dichloromethane/methanol/triethylamine 100:5:1).

MS(ES⁺): 558.3 (M+H)⁺.

Example 59 Preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(4-nitro)imidazolyl)acetyl)glycine

Mmt-Aeg(Im^(4-Nitro))-OH

5.37 g ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(4-nitro)imidazolyl)acetyl)glycinemethyl ester are dissolved in a mixture of 20 ml of methanol, 80 ml ofwater and 13.9 ml of triethylamine. The clear solution is left to standat room temperature for 48 hours. After this period, the startingmaterial has undergone hydrolysis, and the solution is concentrated todryness in vacuo on a rotary evaporator. The residue is subjected to twomore distillations using small amounts of toluene. The residue is takenup in a mixture of in each case 20 ml of dioxane and ethyl acetate, asmall amount of undissolved matter is filtered off, and the filtrate isstirred into 500 ml of ether containing 1% of triethylamine. The productwhich has precipitated is filtered off with suction, washed with a smallamount of ether and dried in a desiccator.

Yield: 4.21 g.

R_(f) =0.24 (dichloromethane/methanol/triethylamine 100:10:1).

MS (ES⁺): 543.2 (M⁺).

Example 60 Preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(1,2,4)-triazolyl)acetyl)glycinemethyl ester

Mmt-Aeg(Triaz)-OMe

To the second half of the above-prepared solution ofMMt-Aeg(chloroacetyl)-OMe in DMF there are added 5.96 g of finelypulverulent 1,2,4-triazole and 23.85 g of finely pulverulent potassiumcarbonate. This mixture is stirred for a further 16 hours at roomtemperature and undissolved matter is then filtered off with suction.The filtrate is concentrated in vacuo on a rotary evaporator, and theresidue is partitioned between water and ethyl acetate. The organicphase is washed three times using in each case 50 ml of water and driedover sodium sulfate, the desiccant is filtered off, and the filtrate isthen concentrated in vacuo on a rotary evaporator to a volume ofapproximately 60 ml. This solution is then stirred into a mixture of 400ml of petroleum ether and 200 ml of diethyl ether, during which processthe product precipitates. The product is filtered off with suction anddried in a desiccator.

Yield: 17.4 g.

R_(f) =0.63 (dichloromethane/methanol/triethylamine 100:5:1).

Example 61 Preparation ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(1,2,3)-triazolyl)acetyl)glycine

Mmt-Aeg(Triaz)-OH

5.13 g ofN-((4-methoxyphenyl)diphenylmethylamino)ethyl-N-((1-(1,2,3)-triazolyl)acetyl)glycinemethyl ester are dissolved in a mixture of 10 ml of dioxane, 10 ml ofmethanol, 80 ml of water and 13.9 ml of triethylamine. The clearsolution is left to stand at room temperature for 48 hours. After thisperiod, the starting material has undergone hydrolysis, and the solutionis concentrated to dryness in vacuo on a rotary evaporator. The residueis subjected to two more distillations using small amounts of toluene.The residue is taken up in a mixture of in each case 20 ml of dioxaneand ethyl acetate, a small amount of undissolved matter is filtered off,and the filtrate is stirred into 500 ml of ether containing 1% oftriethylamine. The crude product which has precipitated (4.02 g) ispurified chromatographically on silica gel using a step gradient (2-15%)of methanol/ethyl acetate (:1) in dichloromethane, with 1% oftriethylamine in all solvents. The product-containing fractions arecombined and concentrated to dryness in vacuo on a rotary evaporator.

Yield: 3.25 g.

R_(f) =0.27 (dichloromethane/methanol/triethylamine 100:10:1).

MS(ES⁺): 500.2 (M+H)⁺.

We claim:
 1. A compound of the formula I or a salt thereof ##STR11## inwhich PG is a trityl type protective group which is labile to weakacids, and is selected from triphenylmethyl (Trt),(4-methoxyphenyl)-diphenylmethyl (Mmt), (4-methylphenyl)-diphenylmethyl(Mtt), di-(4-methoxyphenyl)phenylmethyl (Dmt) and 9-(9-phenyl)xanthenyl(pixyl);X is NH, O or S; Y is CH₂, NH or O, and B' is a base selectedfrom adenine, cytosine, guanine, thymine, uracil, purine,2,6-diaminopurine, 7-deazaadenine, 7-deazaguanine, N⁴ N⁴-ethanocytosine, N⁶ N⁶ -ethano-2,6-diaminopurine, 5-methylcytosine,5-(C₂ -C₆)alkynyluracil 5-(C₂ -C₆)alkynylcytosine, 5-fluorouracil andpseudoisocytosine, wherein any exocyclic amino or hydroxyl group isprotected by a protective group, or B' is further selected fromimidazole, triazole, and nitroimidazole.
 2. A compound of formula I orsalt thereof ##STR12## in which PG is (4-methoxyphenyl)-diphenylmethyl(Mmt) or di-(4-methoxyphenyl)phenyl-methyl (Dmt),X is NH or O, Y is CH₂,and B' is selected from adenine, cytosine, guanine, thymine, uracil,purine, 2,6-di-aminopurine, 7-deazaadenine, 7-deaza-guanine, N⁴ N⁴-ethanocytosine, N⁶ N⁶ -ethano-2.6-diaminopurine, 5-methylcytosine,5-(C₃ -C₆)alkynyluracil, 5-(C₃ -C₆)alkynyl-cytosine, 5-fluorouracil andpseudoisocytosine, wherein any exocyclic amino or hydroxyl group isprotected by a protective group, wherein said protective group for anyexocyclic amino or hydroxyl group is selected from benzoyl, isobutanoyl,acetyl, phenoxyacetyl, 4-(t-butyl)-benzoyl, 4-(t-butyl)phenoxyacetyl,4-(methoxy)-benzoyl, 2-(4-nitrophenyl)ethyloxycarbonyl,2-(2,4-dinitrophenyl)ethyloxycarbonyl, 9-fluorenyl-methoxycarbonyl,diphenylcarbamoyl, and formamidine; or B' is further selected fromimidazole, triazole, and nitroamidazole.
 3. A compound of the formula Ior salt thereof as claimed in claim 1, in whichPG is(4-methoxyphenyl)-diphenylmethyl (Mmt) or di-(4-methoxyphenyl)phenyl-methyl (Dmt), X is NH or O, Y is CH₂, and B' is selected from adenine,cytosine, guanine, thymine, uracil, purine, 2,6-di-aminopurine,7-deazaadenine, 7-deazaguanine, N⁴ N⁴ -ethanocytosine, N⁶ N⁶-ethano-2,6-diaminopurine, 5-methylcytosine, 5-(C₃ -C₆)alkynyluracil,5-(C₃ -C₆)alkynylcytosine, 5-fluorouracil and pseudoisocytosine, whereinany exocyclic amino or hydroxyl group is protected by a protectivegroup, or B' is further selected from imidazole, triazole, andnitroamidazole.
 4. A compound of the formula I or salt thereof ##STR13##in which PG is (4-methoxyphenyl)-diphenylmethyl (Mmt)ordi-(4-methoxyphenyl)phenyl-methyl (Dmt), X is NH or O, Y is CH₂, andB' is guanine, and wherein said guanine is protected by a combination of2-N-acetyl and 6-O-diphenylcarbamoyl.